Membrane and Electrodialysis based Seawater Desalination with Salt, Boron and Gypsum Recovery

ABSTRACT

A method of producing desalinated water and recovering minerals from the feedwater uses membrane separation and electrodialysis brine concentration. This process can recover all of the minerals as high purity industrial minerals, including capturing the calcium and sulfate as agricultural grade gypsum and boron as high purity boric acid. In addition the process allows the use of low cost lime or dolime to produce valuable magnesium hydroxide.

BACKGROUND

1. Field of the Invention

The invention is generally related to desalination of sea water and isspecifically directed to a desalination system adapted for recoveringminerals from the feedwater.

2. Discussion of the Prior Art

Japan has used electrodialysis (ED) to recover NaCl from seawater toproduce edible salt on a large scale for about 40 years. ED plants havebeen installed to recover NaCl from seawater to use in chlor-alkaliplants. The benefit of recovering NaCl from reverse osmosis (RO)concentrate is that the starting salt concentration is twice that ofseawater.

Typically, the energy consumption of salt manufactured with seawaterreverse osmosis (SWRO) rejects the feed to an ED plant is 80% of thatwith seawater as the feed (see: Tanaka, Y., Ehara, R., Itoi, S. andGoto, T. “Ion-exchange membrane electrodialytic salt production usingbrine discharged from a reverse osmosis seawater desalination plant,” J.Membrane Sci. 222, 71-86,2003.2003).

The idea of combining seawater reverse osmosis (RO) and ED to producefreshwater and NaCl is disclosed in U.S. Pat. No. 6,030,535, as well asa zero discharge flow scheme using RO, ED, NF, caustic (NaOH)precipitation of magnesium hydroxide, and evaporation, see for example,U.S. Pat. No. 7,083,730. However neither of these patents addressesgypsum or boron recovery or removal. In addition the silica is notremoved in the ED or NF sections, potentially creating scaling on themembranes when operated with the recycle required for zero discharge.

Gypsum scaling is a well known problem in seawater reverse osmosis (RO)plants and especially in zero discharge desalination plants. Sulfatecomposes approximately 8 wt % and calcium composes 1-1.5 wt % of thetotal dissolved solids in seawater. When concentrated the sulfate andcalcium in the seawater react to form insoluble gypsum.

Concentration occurs in zero discharge or limited discharge plants dueto the extraction of desalinated water and recycle of the brine streamswithout adequate removal of calcium and sulfate. Periodic polarityreversal is used in ED to limit gypsum and calcium carbonate scaling.However, the recycling required for zero discharge can exceed the limitseven with polarity reversal.

U.S. Pat. No. 6,030,535 discloses an ED membrane that is not permeableto sulfate to prevent gypsum formation in the ED concentrate stream.However, significant sulfate and calcium is recycled from the ED diluatestream back to the RO unit potentially creating gypsum scaling on the ROmembranes. A large portion of the low salt content ED diluate (2 wt %dissolved salt) must be taken as a discharge purge back to the sea tolimit the calcium and sulfate concentration in the RO unit brinedischarge stream. It would be desirable to fully utilize the ED diluatestream to produce desalinated water since it contains less salt (2 wt %)than the feed seawater (3.5 wt %).

U.S. Pat. No. 7,083,730 discloses partial soda ash softening of the feedseawater to remove most of the calcium to prevent gypsum scaling.However, this requires a significant amount of caustic and soda ashaddition and produces a mixed calcium carbonate, magnesium carbonatesoftener sludge-for disposal. This increases the operating cost of theplant due to the chemical feed cost, sludge disposal cost and loss ofbyproduct magnesium hydroxide.

The '730 patent provides for valuable magnesium hydroxide production,but due to the presence of sulfate in the magnesium rich brine streamlow cost lime or dolomitic lime cannot be used to precipitate themagnesium hydroxide without contaminating it with gypsum. Caustic (NaOH)must be used which is more expensive. Approximately 1.4 tons of NaOH arerequired to produce 1 ton of Mg(OH)2. Since the market prices for bothNaOH and Mg(OH)2 are typically equal it is not profitable to produceMg(OH)2 using caustic.

Dolomitic lime is used commercially to precipitate Mg (OH)₂ fromseawater. Approximately 0.85 tons of dolomitic lime (CaO+MgO) isrequired to produce 1 ton of Mg(OH)2. The typical market price fordolomitic lime is 30-50% of the price of Mg (OM making its recoveryusing dolomitic lime profitable. Thus it would be economically desirableto develop an enhanced RO and ED based desalination flow scheme thatwould allow the use of dolomitic lime to precipitate the magnesium fromthe seawater brine.

None of the known prior art patent addresses the issue of dissolvedboron build up in the recycle ED diluate stream to the RO unit. Boron inthe form of uncharged boric acid is a component in seawater thattypically must be removed at a 90-95% net rejection from the seawater toproduce drinking water and agriculture water that meets the World HealthOrganization guidelines (0.5 ppm B). Since boron is uncharged this isdifficult to achieve with RO membranes even on a once through basis.

In the ED and NF units the uncharged boron remains with the diluate andpermeate streams and is recycled to the RO unit. This increases itsconcentration several fold in the feed stream to the RO causingbreakthrough into the desalinated product water.

As described in U.S. Pat. Nos. 4,298,442; 5,250,185; and 5,925,255, byraising the pH to 10-10.5 nearly all (>95%) of the uncharged boric acidis converted to monovalent borate and the uncharged silica is convertedto monovalent silicate. This allows the borate and silicate to becaptured in the ED concentrate stream. This prevents build up in therecycle ED diluate and membrane scaling. However, as explained in U.S.Pat. No. 5,925,255 the calcium and magnesium content must be reduced tovery low levels (<<1 ppm) using upstream ion exchange, and bicarbonatemust be essentially removed (<<10 ppm) to prevent CaCO3 or Mg(OH)2scaling. Although U.S. Pat. No. 5,925,255 allows a simple single passremoval of boron and silica with the dissolved salts, it is noteconomically feasible for seawater desalination due to the large volumeof water that would need to be treated and high magnesium and calciumcontent in seawater (>1500 ppm). If the ED feedwater pH were raised to10 a portion of the bicarbonate in the seawater brine would be convertedto carbonate and could form a carbonate scale with the calcium dissolvedin the ED feedwater. In addition magnesium in the seawater could formmagnesium hydroxide and foul the membranes.

U.S. Pat. No. 7,083,730 discloses the use of partial soda ash softeningto remove some of the bicarbonate and calcium. However, significantcalcium and bicarbonate remain in the feed seawater (typically >50 ppmbicarbonate and >100 ppm calcium) along with most of the magnesium. Thisgets concentrated in the RO brine and still could form calcium carbonateor magnesium hydroxide precipitate in the ED unit at pH 10. It would bedesirable to develop an enhanced RO and ED based desalination flowscheme that would allow the ED to operate at a pH of 10 allowing boronand silica rejection without calcium carbonate or magnesium hydroxidescale formation.

In U.S. Pat. No. 7,083,730 boron contamination of the magnesium richbrine is also a problem for magnesium hydroxide production. The presenceof boron is undesirable since it is adsorbed onto the surface of the ofthe magnesium hydroxide particles, making the magnesium hydroxideunsuitable for refractory brick applications. Additional treatment ofthe brine (ion exchange, excess lime addition) is required ifsignificant boron is present. It would be desirable to develop anenhanced RO and ED based desalination flow scheme that would producemagnesium rich brine that has a low boron content allowing high puritymagnesium hydroxide production without additional treatment steps.

Published US Application, Pub. No. 2008/0237123, discloses a method ofregenerating N-Methyl-D-glu-camine-functional resin that has been usedfor boron-removal uses a closed recirculating loop for treating theconjugate acid salt of the N-Methyl-D-glucamine functionality of theresin. The method reduces rinse water demand and improves pH control ina water treatment system, and can be used to improve the performance ofboron-selective resins in stand-alone systems or as a second stage in areverse osmosis seawater desalination system.

An article by Ki-Won Baek, et al, entitled: “Adsorption Kinetics ofBoron by Anion Exchange Resein in Packed Column Bed,” J. Ind. Eng. Chem,Vol. 13, No. 3 (007) 452-456, Discusses an application for Amberlite IRA743 resin that has glucamine functional groups for specific boronexchange. The boron removal efficiency from seawater was examinedthrough the packed column experiment. The efficiency of boron removalwas investigated with respect to several parameters, including the pH ofthe solution, the initial concentration of boron, the bed volume of theresin, and the temperature. The performance of boron removal increasedupon increasing the batch ratio of resin to boron and decreasing theinitial concentration of boron in solution. The removal rate of boron isindependent of temperature. Most of the boron in aqueous solution couldbe removed efficiently under the optimum operation conditions at pH 8.5.In addition, a kinetic study of boron removal under the optimumconditions fit Thomas's adsorption model well.

Many of the seawater RO plants currently in operation discharge theentire reject brine stream with no minerals recovery. It would bedesirable to retrofit these plants with an ED based system that allowseconomic recovery of the minerals. However, the prior art systemsdescribed here recycle all or a portion of the ED diluate stream back tothe RO unit. This recycle increases the flow and changes the compositionof the RO feedwater. This requires modifications to an existing RO baseddesalination facility requiring an extended outage. In addition thechange in feedwater composition may cause unforeseen scaling problems inthe existing RO unit.

It would be desirable to develop an enhanced RO and ED baseddesalination flow scheme that could process the brine stream from anexisting RO desalination plant without impacting the operation of theexisting RO desalination plant.

SUMMARY OF THE INVENTION

The subject invention provides a method of producing desalinated waterand recovering minerals from the feedwater using membrane separation andelectrodialysis brine concentration. This process can recover all of theminerals as high purity industrial minerals, including capturing thecalcium and sulfate as agricultural grade gypsum and boron as highpurity boric acid. In addition the process allows the use of low costlime or dolime to produce valuable magnesium hydroxide.

In one aspect of the invention a standalone system is described.Seawater or brackish water is routed to a solids removal pretreatmentsection which uses a combination of settling, filtration, andultra-filtration to produce a feedwater stream essentially free ofsuspended solids. The pH of the feed water is adjusted to pH 6-6.5 aspart of the pretreatment. This minimizes soluble silica in the feedwaterto prevent downstream membrane fouling. Coagulants, flocculants andperiodic disinfectant addition may be desired in the pretreatmentsection depending on the feedwater quality. The pretreated feed water ispreheated using low level heat to 85-113° F. and additional hydrochloricor sulfuric acid is added to reduce the pH to 4, converting essentiallyall of the bicarbonate to free CO₂. A multistage air or nitrogenstripper is then used to reduce the CO₂ content in the feed water tobelow 5 ppm. Mol sieves may be optionally used to reduce the CO₂ contentin the stripping air.

Sodium bisulfite is added to the stripped feed water to neutralize anyresidual hypochlorite, or hypobromite and the water is fed to ananofiltration unit. The nanofiltration unit removes nearly all (90-99%)of the magnesium, calcium, and sulfate in a non-permeate stream andremoves very little of the boron. This stream is processed for mineralsrecovery to produce agricultural grade gypsum from the. Excess calciumis removed from the NF brine stream as a calcium chloride brine, flakeor pellet product.

The NF permeate and the purge from the NF non-permeate minerals recoverysystem containing the monovalent salts is adjusted to pH 9-10 usingcaustic and routed to a seawater RO system. The elevated pH andtemperature allow optimized flux and boron removal with low pressuredrop RO membranes. Multiple stages of permeate and nonpermeate RO may beused depending on the feed water quality and desal water purityrequirement.

The reject brine stream from the seawater RO system, preferably with70-80 g/l Total Dissolved Solids (TDS), is routed to an electrodialysisor electrodialysis reversal (ED(R)) unit which operates at pH 9.5-10.The elevated pH converts most of the uncharged boric acid to monovalentborate, and the uncharged silica to silicate, allowing the ED(R) unit tocapture most of these ions in the concentrate stream. The ED(R) diluatestream, preferably with 20 g/l TDS, is pH adjusted to 9.5-10 usingcaustic and routed to a brackish water RO system which producesdesalinated water.

The elevated pH and temperature allow optimized flux and boron removal.The brackish water RO membrane allows a lower pressure drop and higherrecovery than the seawater RO membranes, since it is optimized for thelower TDS in the feed and reject brine.

The reject brine from the brackish water RO system is recycled back tothe ED(R) unit. Silica and boron do not build up in the recycle loopsince most of the silica and boron fed to the ED(R) is rejected with theconcentrate due to the elevated pH.

ED(R) concentrate preferably with 230 g/l TDS is routed to a brinestorage tank which facilitates off-peak salt production from theconcentrate using mechanical vapor recompression (MVR) evaporators andcrystallizers. The brine storage also allows the desalination plant tocontinue to operate when the downstream MVR units experience a forced orplanned outage. Brine storage is also provided within the NFnon-permeate minerals recovery system to allow desalination plantoperation when one of the components in the NF non-permeate mineralsrecovery system is down.

The ED(R) concentrate stream from the brine tank is processed forminerals recovery. This produces high purity salt and recovers boron ashigh purity boric acid.

Another aspect of the invention permits incorporation of the methodologyin a retrofit operation. In this case the system is retrofitted on allor a portion of the reject brine stream of an existing desalinationplant. The reject brine feed stream is adjusted to pH 6-6.5 usinghydrochloric acid, sulfuric acid or caustic and is filtered (cartridgefilter, or ultrafiltration) to remove any suspended solids. The residualfilter purge solid stream is routed back to the RO unit solids disposalline or to a separate solids settling and filtration system producingbyproduct solids. The pH adjustment minimizes soluble silica in thefeedwater to prevent downstream membrane fouling. The feed water streamis processed the same as the standalone embodiment except the seawaterRO block is not used. The elevated TDS NF permeate stream is feddirectly to the ED(R) unit. The flow scheme downstream of the ED(R) unitis the same as the standalone embodiment. No streams are recycled to theexisting desalination plant.

Another embodiment of this invention is used to produce a salt brinewith a further reduced calcium and magnesium content, using higherefficiency ED membranes for the bulk of the salt concentration.

The subject invention provides numerous advantages over prior artsystem, including, but not limited to:

1) Production of high quality desalinated water from ED(R) diluate atlower energy consumption than seawater RO membranes by using lowpressured brackish water RO at pH 9-10.

2) Zero discharge seawater desalination without buildup of boron whichcan cause contamination of the desalinated water.

3) Zero discharge seawater desalination without buildup of silica whichcan cause membrane scaling and fouling.

4) Does not require specialty monovalent ED(R) membranes.

5) Can be used to as a retrofit system on an existing RO plant.

6) Desalinated water low in boron is produced with the option to recoverboron as byproduct boric acid.

7) Sulfate is recovered as an Agricultural grade gypsum byproduct.

8) Magensium is recovered as high purity magnesium hydroxide (withoutboron or gypsum contamination) using low cost dolime to precipitate themagnesium hydroxide.

9) Calcium carbonate, magnesium hydroxide softener sludge is notproduced. Essentially all of the calcium is recovered as eitheragricultural gypsum or byproduct calcium chloride.

Novel Features

The novel features of the invention include but are not limited to:

1) Uses a UF, acidic air stripper and NF pretreatment system forseawater or RO brine to produce a low carbonate<<5 ppm), 10 wCa<<200ppm), low Mg<<100 ppm) brine ED(R) feed stream at pH 9.5-10 allowing theED to reject the majority of the boron and silica preventing build up inthe recycle stream.

2) Uses a dedicated low pressure drop brackish water RO at pH 9-10 toproduce high quality desalinated water from ED(R) diluate, and a brinestream suitable for recycle to the ED(R) unit.

3) Uses the gypsum, magnesium hydroxide, calcium chloride recoverysystem described in my copending patent application Ser. No. ______,filed on Jun. 18, 2010, entitled: “Zero Discharge Water DesalinationPlant with Minerals Extraction Integrated with Natural Gas CombinedCycle Power Generation”, to recover these minerals without passing themthrough the ED(R).

4) Produces a low sulfate, low calcium, low magnesium concentrated brinestream (20 wt % dissolved solids) suitable for production of salt or usein the NaCl, KCl, Br₂, boric acid recovery system in Katana's previousdesalination patent.

Additional Novel Features relating to the Calcium and Magnesium PurgeConfiguration:

Utilizes a multistage ED and RO configuration operating at various pH'swith various combinations of mono and divalent permselective membranesto separate an RO brine into the three following streams, without scaleproduction:

1) high purity, high concentration product salt brine.

2) high concentration calcium chloride, magnesium chloride purge brine,

3) high purity product desalinated water.

Other advantages, objects and features of the invention will be readilyapparent from the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a standalone desalination system utilizing EDR-ROintegration for mineral recovery.

FIG. 2 is a detail of the reverse osmosis (RO) loop.

FIG. 3 is a diagram of an alternative standalone desalination systemincorporating mineral recovery.

FIG. 4 is an example diagram for incorporating the mineral recoverysystem of the subject invention is an existing desalination plant.

DETAILED DESCRIPTION

The following abbreviations and acronyms are used throughout theapplication:

BWRO: brackish water reverse osmosis

ED: electrodialysis

ED(R): electrodialysis reversal

g/l: grams per liter

MGD: million gallons per day

MVR: mechanical vapor recompression

NF: nano filtration

RO: reverse osmosis

SWRO: seawater reverse osmosis

sTPD: short tons per day

TDS: total dissolved solids

Standalone Systems

Referring to FIG. 1 of the drawings, seawater or brackish water at input12 is routed to a solids removal pretreatment section 10 which uses acombination of settling, filtration, and ultra-filtration to produce afeedwater stream essentially free of suspended solids. The pH of thefeed water is adjusted to pH 6-6.5 as part of the pretreatment processusing hydrochloric acid, sulfuric acid or caustic as necessary, asindicated at 14. The pH adjustment minimizes soluble silica in thefeedwater to prevent downstream membrane fouling. Coagulants,flocculants and periodic disinfectant (NaOCl, chlorine) addition may berequired in the pretreatment section depending on the feedwater quality.Solid amendment/landfill solids are released as indicated at 16. Thepretreated feed water is preheated using low level heat to 85-113° F.(preferably above 100° F.) and additional hydrochloric or sulfuric acidis added as indicated at 24 to reduce the pH to 4 converting essentiallyall of the bicarbonate to free CO2.

A multistage air or nitrogen stripper 22 is then used to reduce the CO₂content in the feed water to below 5 ppm (preferably below 1 ppm). Molsieves may be optionally used to reduce the CO₂ content in the strippingair.

Sodium bisulfite is added to the stripped feed water, as indicated at26, to neutralize any residual hypochlorite, or hypobromite and thewater is fed to a nanofiltration unit 28. The nanofiltration unitremoves nearly all (90-99%) of the magnesium, calcium, and sulfate in anon-permeate stream, as indicated at 30 and removes very little of theboron. This stream is processed for minerals at the mineral recoveryunit 32. An example of a mineral recovery system is shown and describedin my copending patent application Ser. No. ______, filed on Jun. 18,2010, entitled: “Zero Discharge Water Desalination Plant with MineralsExtraction Integrated with Natural Gas Combined Cycle Power Generation”,which is fully incorporated by reference herein. The mineral recoveryunit, or MVR 32, produces agricultural grade gypsum from the sulfate anduses lime or dolime to precipitate high purity magnesium hydroxide. TheMVR 32 comprises a non-selective anion and cation membrane with optionaldownstream diluate RO. Excess calcium is removed from the NF brinestream as a calcium chloride brine, flake or pellet product. The releasestream 33 from MVR 32 comprises condensate at 2 MGD.

The NF permeate at 34 and the purge from the NF non-permeate mineralsrecovery system at 36 containing the monovalent salts is adjusted to pH9-10 using caustic and routed to a seawater RO system 38. The elevatedpH and temperature allow optimized flux and boron removal with lowpressure drop RO membranes. Multiple stages of permeate and nonpermeateRO may be used depending on the feed water quality and desal waterpurity requirement.

The reject brine stream from the SWRO system at 40, preferably with70-80 g/l TDS, is routed to an electrodialysis or electrodialysisreversal ED(R) unit 42, which operates at pH 9-10. The elevated pHconverts most of the uncharged boric acid to monovalent borate, and theuncharged silica to silicate, allowing the ED(R) to capture most ofthese ions in the concentrate stream. The ED(R) diluate stream, at 44,preferably with 20 g/l TDS is pH adjusted to 9-10 using caustic androuted to a BWRO system 46, which produces desalinated water. Therelease stream 39 from SWRO 38 is RO permeate at 18 MGD.

The elevated pH and temperature allow optimized flux and boron removal.Multiple stages of permeate and non-permeate may be used depending onthe feed water quality and desal water purity requirement. The BWROmembrane allows a lower pressure drop and higher recovery than the SWROmembranes, since it is optimized for the lower TDS in the feed andreject brine.

The reject brine from the BWRO system is recycled back to ED(R) unit, asindicated at 48. Silica and boron do not build up in the recycle loopsince most of the silica and boron fed to the ED(R) 42 is rejected withthe concentrate due to the elevated pH.

ED(R) concentrate at 50, preferably with 230 g/l TDS, is routed to abrine storage tank 52 which facilitates off-peak salt production fromthe concentrate using mechanical vapor recompression (MYR) evaporatorsand crystallizers. The brine storage also allows the desalination plantto continue to operate when the downstream MVR units 54 experience aforced or planned outage. Brine storage is also provided within the NFnon-permeate minerals recovery system to allow desalination plantoperation when one of the components in the NF non-permeate mineralsrecovery system is down.

The ED(R) concentrate stream at 56, from the brine tank, is processedfor minerals recovery as explained in my aforementioned co-pendingapplication Serial No. ______. The MVR 54 produces high purity salt andrecovers boron as high purity boric acid.

The RO detail is shown in FIG. 3. The NF permeate at 34 is introducedinto the RO 38 as indicated in FIGS. 1 and 2, and is at a temperature ofabout 104° F. The RO 38 typically comprises the SWRO 90, the BWRO 96 andthe SWRO 98. The stream at 34 is at pH 6 which is mixed with caustic andrecycle stream 100 to increase the pH to 8-9. This is introduced into RO90 which operates at pH 8-9. The primary release stream 92 isdesalinated water. The secondary stream 93 is at pH 8-9 and isintroduced into SWRO 98. The outlet stream 99 is combined with stream 92and introduced into BWRO 96, which is at pH 10. A bypass loop 94 is alsoprovided. The outlet stream from BWRO is at 39, as also indicated inFIGS. 1 and 2 as the outlet from SWRO system 38. Stream 100 from BWRO 96is reintroduced into stream 34. Outlet stream 40 is introduced into ED42 in FIG. 1 and brine tank 52 in FIG. 2.

Retrofit System

Another embodiment of this invention is used as a retrofit on all or aportion of the reject brine stream of an existing desalination plant, asshown in FIG. 4. The reject brine feed stream 110 is adjusted to pH6-6.5 using hydrochloric acid, sulfuric acid or caustic and is filtered(cartridge filter, or ultrafiltration) to remove any suspended solids,as indicated at the pretreatment unit 10. Then, as with the previouslydescribed embodiments, stream 18 is heated at preheater 20 andintroduced into the air stripper 22. The NF 28, MVR 32, ED 42, BWRO 46,brine tank 52 and MVR 54 perform as previously described.

The residual filter purge solid stream is routed back to the RO unitsolids disposal line (not shown) or to a solids settling and filtrationsystem included in the pretreatment section that produces byproductsolids. The pH adjustment minimizes soluble silica in the feedwater toprevent downstream membrane fouling.

The feed water stream is processed the same as the standalone embodimentexcept the seawater RO block 38 is not used. The elevated TDS NFpermeate stream 34 is fed directly to the ED(R) unit 42. The flow schemedownstream of the ED(R) unit is the same as the standalone embodiment.No streams are recycled to the existing desalination plant.

Calcium and Magnesium Purge System

Another embodiment of this invention is used to produce a salt brinewith a further reduced calcium and magnesium content, using higherefficiency ED membranes for the bulk of the salt concentration, andfollows the configuration of FIG. 2.

This embodiment is substantially identical to the embodiment of FIG. 1from the sea water intake at 12 through the SWRO unit 38. However, inthis embodiment the reject brine stream 40 from the SWRO 38 is firstintroduced into the brine tank 52 and from there into ED 60, whichcomprises a monovalent selective cation membrane for Na and K passageand a non-selective anion membrane for Cl, Br, B(OH)₄, CO3, and SO4passage. This releases a stream 62 at 7.7 MGD and 20 g/l TDS into the RO64, and a second stream 80 at 2.5 MGD and 270 g/l TDS for introductioninto the MVR 54. The RO produces a stream at 66 which is at 1.9 MGD and70 g/l TDS. This is introduced into a series of ED's 68, 72 and 76. Therelease streams from these ED's is as follows:

ED 68: stream 70 at 1.5 MGD and 15 g/l TDS; stream 71 at 0.4 MGD and 270g/l TDS.

ED 72: stream 74 at 1.2 MGD and 70 g/l TDS; stream 75 at 0.35 MGD and270 g/l TDS.

ED 76: stream 77 at 1.1 MGD and 0.2 g/l TDS; stream 78 at 0.1 MGD and 10g/l TDS.

Stream 75 is fed back into the first minerals recovery system 32.

Streams 71 and 78 are combined with stream 80 from ED 60 to formcombination stream which includes Na, K, SO₄, HCO₃, B(OH)₄, and is at apH 6-7. This is introduced into the MVR 54 for recovering high puritysalt at approximately 3300 s TPD, high purity potassium chloride at 100s TPD, high purity bromine at 8 s TPD and high purity Boric acid at 2 sTPD. The release stream 55 from MVR 54 comprises condensate at 3.5 MGD.

ED 60, and ED 68 comprise a monovalent selective cation membrane forpassing Na and K and a non-selective anion membrane for Cl, SO4, HCO₃,Br passage. ED 72 comprises a monovalent selective anion membrane for Clpassage and a non-selective cation membrane for Na, Ca and Mg passage.ED 76 comprises a non-selective anion and cation membrane with optionaldownstream diluate RO (with the RO brine recycled back to ED).

First Stage ED 60:

The reject brine 40 from the RO unit is pH adjusted with HCl to pH 8 andis fed to a brine tank 52 that is filled during on-peak power priceconditions (typically 8 hours per day) and emptied during off-peak powerconditions. During off-peak power conditions RO brine from the tank isfed to the first stage ED 60 which is equipped with monovalent cationpermselective membranes and monodivalent anion permselective membranes.The first stage ED 60 is operated at pH 8-8.5 which allows higherefficiency anionic ED membranes, which are not compatible with high pHoperation. Most (>85-98%) of the calcium and magnesium, boric acid andsilica remain in the diluate stream. Most (70-90%) of the salt,potassium chloride, sodium bromide are extracted into the brineconcentrate stream.

The first stage ED brine concentrate is routed to an evaporation sectionwhere salt, potassium chloride, bromine and boric acid are recoveredfrom the brine. The first stage ED diluate is pH adjusted to pH 8 withHCl and is routed to a two stage RO unit.

Two Stage RO Unit 64:

The first stage of RO 38 removes essentially all (>99%) of the dissolvedsalts (NaCl, KCl, NaBr, CaHCO₃, MgHCO₃) and most of the boric acid(75-85%). The permeate from the first stage is pH adjusted to pH 10 withcaustic and fed to the second stage RO. Operation of the second stage atpH 10 enables near complete removal (98%) of the residual boric acid assodium borate. Calcium or magnesium scale does not form in the secondstage due to the low residual concentrations of calcium, and magnesium.The second stage RO brine stream 66 is pH adjusted to 8 with HCl andrecycled to the first stage RO 38. The second stage RO permeate 65 isrouted to the desalinated product water.

The first stage RO brine is fed to the second stage ED unit 68. Thefirst and second stage RO units remove approximately 65% of the waterfrom the first stage ED diluate as desalinated product water. Duringon-peak operation the primary ED unit does not produce diluate (internalrecirculation only) and a portion (10-20%) of the seawater RO feed isrouted to the RO unit. This minimizes on-peak RO power consumption dueto the increased membrane area, and enables continuous operation of bothseawater and ED diluate RO units.

Second Stage ED 68:

The first stage RO brine at pH 8 is fed to the second stage ED 68 whichuses the same membrane types as the first stage ED 60. The second stageED concentrate brine is also routed to the evaporation section with thefirst stage ED brine.

Third Stage ED 72:

The second stage ED diluate 70 is routed to the third stage ED unit 72which also operates at a pH 8 and uses monodivalent cation permselectivemembranes and mono anion permselective membranes. The third stage EDconcentrate stream contains calcium, magnesium, and sodium chloride isrouted to the magnesium and calcium recovery section of the plant. Thediluate stream from the third stage ED diluate stream containing sodiumchloride and residual sulfate, boron (as undissociated boric acid), andsilica is pH adjusted to 9.5-10.5 using caustic and routed to the fourthstage ED unit.

Fourth Stage ED 76:

The fourth stage ED unit 76 uses monodivalent cation permselectivemembranes and monodivalent anion permselective membranes to produce adesal product water diluate stream. Optionally the desal product waterdiluate can be routed to a polishing RO unit to further reduce thesodium borate content in the product desalinated water, with the RObrine recycled back to the fourth stage ED unit 76. The fourth stage EDconcentrate stream 78 containing mainly sodium chloride, sodium borateand sodium silicate is routed to the evaporation section, MVR 54.

Optionally an ion exchange resin operating at pH 9-10 (not shown in FIG.2) can be added upstream of the fourth stage ED to capture boric acidand the silica rich fourth stage ED concentrate stream 78 recycled tothe inlet pretreatment section. The boric acid purge stream from the ionexchange resin is pH adjusted to 6-7 with NaOH and fed separately intothe minerals recovery section for boric acid recovery, as explained inmy aforementioned co-pending application Ser. No. ______. This reducesthe silica and boric acid loading in the MVR and increases the saltcontent in the MVR feed.

Key Benefits

The key benefits of this invention are summarized below:

1) Uses a dedicated low pressure drop brackish water RO at pH 9-10. toproduce high quality desalinated water from ED(R) diluate at lowerenergy consumption than seawater RO membranes.

2) Provides zero discharge seawater desalination without buildup ofboron which can cause contamination of the desalinated water.

3) Provides zero discharge seawater desalination without buildup ofsilica which can cause membrane scaling and fouling.

4) Does not require specialty monovalent ED(R) membranes.

5) Can be used to as a retrofit system on an existing RO plant. Thesystem would divert all or a portion of the RO reject brine stream fromdisposal to an ED(R)/BWRO based system that uses an acidic airstripping, NF and Katana NF brine mineral recovery system to pretreatthe ED(R) feed.

6) Produces desalinated water low in boron with the option to recoverboron as byproduct boric acid.

7) Recovers sulfate as an agricultural grade gypsum byproduct.

8) Recovers essentiality all of the magnesium as high purity magnesiumhydroxide (without boron or gypsum contamination) using low cost dolimeto precipitate the magnesium hydroxide.

9) Does not produce a mixed calcium carbonate, magnesium hydroxidesoftener sludge for disposal. Essentially all of the calcium isrecovered as either agricultural gypsum or byproduct calcium chloride.

Additional Benefits of the Calcium and Magnesium Purge Embodiment

1) Produces a salt brine with a lower CaCl₂ and MgCl₂ content whichreduces the boiling point elevation in the salt crystallizer. Thisreduces the capital cost and power consumption of the salt crystallizer.The salt brine can also be cost effectively polished before evaporationwith cation exchange resins to essentially eliminate the residual Ca andMg from the salt brine. This enables the production of high value, highpurity, chemical grade salt (>99.995% purity).

2) Uses a mono and divalent anion permselective ED membranes andmonovalent cation permselective ED membranes which reduces membraneresistance by 50% versus the monoselective anion and cationpermselective ED membranes typically used for salt brine production.This also reduces gypsum and calcium carbonate fouling since sulfate,bicarbonate and carbonate is directed to the concentrate and calcium isdirected to the diluate, preventing calcium sulfate, or calciumcarbonate concentration increase and gypsum or carbonate scaling in theED unit.

3) Enables economical use of boric acid ion exchange resin at optimal pHof 9-10 on a small stream (<5% of feed) with elevated boric acid content(20 times feed seawater). This concentrates the boric acid into a highconcentration (2 wt % boric acid) small 0.04 MGD purge stream suitablefor boric acid recovery.

While certain features and embodiments of the invention have beendescribed in detail herein it should be understood that the inventionencompasses all modifications and enhancements within the scope andspirit of the attached claims.

1. A method for salvaging minerals from seawater comprising: a.Preheating seawater; b. Passing the preheated seawater to through a nanofiltration unit to separate the seawater into a non-permeate and apermeate; c. Capturing the minerals in the non-permeate from the nanofiltration unit in a first stage recovery; d. Passing the permeate fromthe nano filtration unit to a second stage recovery comprising anelectrodialysis step to generate a concentrate; e. Capturing theminerals in the concentrate from the electrodialysis step.
 2. The methodof claim 1, further including the step of receiving the seawaterdischarged from the electrodialysis step in a brackish water reverseosmosis step for producing a brackish water permeate.
 3. The method ofclaim 1, including the step of passing the preheated seawater through anair stripper prior to introduction into the nano filtration unit.
 4. Themethod of claim 1, wherein HCl is added to the seawater in apretreatment step in advance of the preheating step.
 5. The method ofclaim 4, wherein the pH of the seawater is adjusted to a Ph 6.5 in thepretreatment step.
 6. The method of claim 1, including the step ofintroducing NaHSO₃ in advance of the nano filtration unit to adjust thepH of the seawater to pH 6 in advance of the nano filtration unit. 7.The method of claim 1, including the step of introducing the concentratefrom the electrodialysis system into a brine tank before capturing theminerals.
 8. The method of claim 1, wherein the first stage mineralrecovery utilizes dolime as an active agent.
 9. The method of claim 1,wherein gypsum, magnesium hydroxide and calcium chloride are recoveredin the first stage.
 10. The method of claim 1, wherein high purity salt,potassium chloride, bromine and boric acid are recovered in the secondstage.
 11. The method of claim 1, including the step of passing theseawater from the nano filtration unit through a seawater reverseosmosis system prior to introducing it into the electrodialysis step.12. The method of claim 1, including the step of passing the concentratefrom the electrodialysis step through a brackish water osmosis systemand reintroducing the non-permeate therefrom into the electrodialysisstep.
 13. The method of claim 2, wherein the seawater is preheated to atemperature of 85-113° F.
 14. A method for salvaging minerals fromseawater comprising: a. Preheating seawater; b. Passing the preheatedseawater to through a nano filtration unit to filter the seawater intopermeate and non-permeate; c. Passing the permeate from the nanofiltration unit through a seawater reverse osmosis system d. Capturingthe minerals in the non-permeate from the nano filtration unit from thenano filtration unit in a first stage recovery; e. Passing the permeatefrom the nano filtration unit to a second stage recovery comprising anelectrodialysis step; f. Capturing in the second stage recovery theminerals in the permeate from the nano filtration unit.
 15. The methodof claim 14, further including the step of receiving the seawaterdischarged from the electrodialysis step in a brackish water reverseosmosis step for producing a brackish water permeate.
 16. The method ofclaim 14, including the step of passing the preheated seawater throughan air stripper prior to introduction into the nano filtration unit. 17.The method of claim 14, wherein HCl is added to the seawater in apretreatment step in advance of the preheating step.
 18. The method ofclaim 17, wherein the pH of the seawater is adjusted to a Ph 6.5 in thepretreatment step.
 19. The method of claim 14, including the step ofintroducing NaHSO₃ in advance of the nano filtration unit to adjust thepH of the seawater to pH 6 in advance of the nano filtration unit. 20.The method of claim 1, including the step of introducing the concentratefrom the electrodialysis unit into a brine tank before capturing theminerals.
 21. The method of claim 1, wherein the first stage mineralrecovery utilizes dolime as an active agent.
 22. The method of claim 1,wherein gypsum, magnesium hydroxide and calcium chloride are recoveredin the first stage.
 23. The method of claim 1, wherein high purity salt,potassium chloride, bromine and boric acid are recovered in the secondstage.
 24. The method of claim 1, including the step of passing theconcentrate from the electrodialysis step through a brackish waterosmosis system and reintroducing the discharge therefrom into theelectrodialysis step.
 25. The method of claim 2, wherein the seawater ispreheated to a temperature of 85-113° F.
 26. A method for salvagingminerals from seawater comprising: a. Preheating seawater; b. Passingthe preheated seawater to through a nano filtration unit to filter theseawater into a permeate and a non-permeate; c. Passing the permeatefrom the nano filtration unit through a seawater reverse osmosis systemd. Capturing the minerals in the non-permeate from the nano filtrationunit in a first stage recovery; e. Passing the non-permeate from thenano filtration unit to a second stage recovery comprising: i. A brinetank; ii. An initial stage electrodialysis step; iii. A reverse osmosisstep; iv. A secondary an electrodialysis step; v. A third electrolysisstep; vi. A fourth electrolysis step; f. A mineral recovery step.